In the paint industry it is generally customary in the production of pigmented paints to grind the pigments in some of the binder to be used in formulating the paints or in special binders (paste resins) which are compatible with the paint components. To achieve adequate corrosion resistance on non-pretreated sheet steel, it has been necessary to use lead compounds and/or lead pigments which in the majority of cases are incorporated into these pigment resins together with other pigments and extenders.
However, all lead compounds currently used for such purpose have considerable drawbacks for cathodically depositable electrocoating paints. The lead compounds used as solids (lead oxide and basic lead silicate) must first be dissolved in the paint. Consequently, the paints require longer homogenization times before the full effectiveness of the lead salt as catalyst is attained. The water-soluble salts formed in the process--as well as other water-soluble salts added directly to the paint--are located to a large extent not in the resin micelle, but in the aqueous phase of the paint. They are washed out of the film during electrodeposition by the endosmosis effect occurring in the film and cause water removal from the film, or they are deposited on the substrate in the form of a metallic precipitate. In addition, dissolved salts can be partly removed from the bath by the dialysis equipment used in the electrocoating plant and are possibly returned to the bath in an uncontrolled manner during the rinsing operations.
The water-insoluble lead salts of longer-chain fatty acids are soluble in the resin and, therefore, remain to a considerable extent in the resin micelle. However, titratable amounts of water-insoluble fatty acids which interfere with the deposition characteristics of the paint film and the bath operation during the electrocoating process are formed by hydrolysis.
Furthermore, the use of lead compounds in paints is being increasingly rejected by the industry because of the toxicity of these compounds and the waste disposal problems associated with them. On the other hand, however, adequate corrosion protection of bare metal represents an essential requirement of the users of cathodically deposited electrocoating paints, particularly in the automotive industry.
EP 336,283 A2, corresponding to commonly assigned U.S. Ser. No. 334,526 filed Apr. 7, 1989, describes basic organotitanate compounds as additives for cathodically depositable paint systems, by means of which the corrosion protection of bare metals is significantly improved. This method requires, however, the preparation and storage of an additional paint component. Pigment grinding in such additives is not possible owing to the marked structural viscosity of these compounds.
U.S. Pat. No. 4,973,613, commonly assigned, corresponding to EP 347,785 A2, relates to cathodically depositable electrocoating paints which comprise combinations of cationic film-forming resins with organic titanium compounds, the latter being obtained by reacting a tetraalkyl orthotitanate and/or a titanium acetylacetonate with NH-functional .beta.-hydroxyalkylamines, followed by reaction with formaldehyde.
It has now been found that it is possible to produce cationic paint binders for formulating pigment pastes for cathodically depositable electrocoating paints on the principle of the reaction scheme outlined in U.S. Pat. No. 4,973,613, if epoxy resin-amine adducts with a specific structure are used.